For many years, electrical cables have been insulated to avoid short-circuits. In recent decades, the insulation has been of plastic material, generally applied by an extruder in a known extrusion process in which a conductor is continuously advanced into the extrusion apparatus.
During the application of insulation to conductors by extrusion, certain conductor defects inevitably generate problems which cause conductor breakage or otherwise stop the extrusion process and interrupt production of the insulated conductor. Such production interruptions have occurred for many years and have resulted in uncounted wasted material and lost production time. In particular, a progressive defect known as a "birdcage" is a frequent occurrence and, if the extrusion apparatus is not promptly brought to a stop when a "birdcage" defect first appears, the conductor will break in the extruder, resulting in lost time restringing the conductor through the extruder and then restarting the production machinery. Cable conductor kinks are similarly problematical. Additional time and labor is required to return the system to normal operation producing acceptable cable at an acceptable production rate.
Investigation of the problem has revealed that, following the initiation of a "birdcage" defect, in which the wire cross-sectional diameter is increased, the conductor generally broke completely about 7 to 10 seconds after the "birdcage" defect first began. It was also discovered that certain extrusion machines could be shut down before conductor breakage occurred if the conductor defect could be detected sufficiently early in the progression of the defect. Typically, an emergency stop requires 4 to 5 seconds after emergency stop activation for the extrusion machinery to be completely shut down and stopped. Even the most alert extrusion machine operators cannot ordinarily shut down the machine in time to avoid conductor breakage.